Stars hop BETWEEN GALAXIES
Stellar interlopers could be speeding towards us at huge velocity
A group of astronomers looking for stars escaping the Milky Way could instead have found several invading it from another galaxy.
The team made the discovery during a recent analysis of data from the Gaia spacecraft. Since 2014, Gaia has measured the positions, parallaxes (a measure of distance) and 2D proper motion on the sky of over a billion stars. Seven million of these stars have full 3D data, meaning it’s possible to work out how quickly they are moving towards or away from us.
A team of researchers from Leiden University in the Netherlands used this 3D data to search for hypervelocity stars. These are stars that have been accelerated to phenomenal speeds, probably through interactions with a supermassive black hole.
“Of the seven million Gaia stars with full 3D velocity measurements, we found 20 that could be travelling fast enough to eventually escape the Milky Way,” says Elena Maria Rossi, one of the researchers involved in the study.
Based on a previous look at the Gaia data, the team expected to find perhaps one hypervelocity star breaking loose from the Galaxy, so were pleasantly surprised when so many turned up. More unexpected was the direction in which the stars were travelling.
“Rather than flying away from the Galactic centre, most of the high velocity stars we spotted seem to be racing towards it,” says Tommaso Marchetti, who took part in the research. “These could be from another galaxy, zooming right through the Milky Way.”
The stars may have originated in the Large Magellanic Cloud or they could come from much further afield. Astronomers will now study the stars to see if they can determine their origin. If they do turn out to be intergalactic interlopers, the stars could offer researchers a unique opportunity to take a detailed look at stars from another galaxy.
“Looking at the colours of the stars tells us what they are made of,” says Marchetti. “A star from the Milky Way halo is likely to be fairly old and mostly hydrogen, whereas stars from other galaxies could contain lots of heavier elements.” sci.esa.int/gaia